Entamoeba histolytica (Eh) is a protozoan intestinal parasite that infects 480,000,000 people, and causes more than 50,000,000 cases of diarrhea, and 50,000 deaths yearly. Despite the clinical importance of this organism little is known about the biology, pathogenic mechanisms, or genetics of Eh. Recently we used differential screening of an Eh cDNA library to clone the gene encoding an unique Eh surface membrane antigen, serine rich Entamoeba histolytica protein (SREHP). There are a number of features of SREHP which suggest it may have relevance in studying Eh biology, and in the development of clinical reagents for the prevention of amebiasis. The overall structure of SREHP resembles the structure of the malarial circum sporozoite proteins, in that both contain signal sequences, a region of charged amino acids, multiple tandem repeats, and a hydrophobic anchor region. The native SREHP molecule undergoes post-translational modifications which include glycosylation, phosphorylation, and acylation. SREHP is naturally immunogenic, and appears to have conserved epitopes, based on serological studies from patients with invasive amebiasis. Preliminary studies suggest SREHP may play a role in amebic adhesion to target cells. We propose to extend our studies of SREHP by determining the fine structure of the native SREHP molecule with the goal of better understanding the nature of post-translational modifications in Eh, and structure/function relationships for SREHP. The role of SREHP in Eh - target cell interactions will be investigated by direct binding studies using recombinant SREHP, and by the selection of SREHP (-) variants. We will characterize the nature of the immune response to SREHP, and determine whether immunization with SREHP can protect against amebiasis in animal models. These studies should provide further insights into the immune response to Eh antigens, the design of vaccines to prevent invasive amebiasis, and the biology of Eh.